Smelting of tin



United States Patent SMELTING F TIN Stephen Esslemont Woods, Bristol, England, assignor, by

No Drawing. Application February 3, 1954, Serial No. 408,043

1 Claim. (Cl. 75--85) Claims priority, application Great Britain February 6, 1953 The present invention relates to the melting of tin.

It is known to smelt tin ores in a blast furnace, the tin being obtained from the bottom of the furnace.

It has now been discovered that under special conditions tin can be obtained from the vapour oiftake of a blast furnace.

The invention consists of a method of smelting tin ores in which the tin ores are smelted in a blast furnace to gether with zinc ores or in the form of combined tinzinc ores and the tin is recovered together with zinc from the vapour offtake of the furnace.

An example of the application of the invention will be described.

1100 lb. of a sintered material containing 1.2% zinc and 18.2% tin by weight was charged with 4400 lb. of a sintered material containing 55.4% zinc by weight, 1500 lb. of another material containing 40.0% zinc by weight, 5400 lb. of metallurgical coke, 1500 lb. of burnt lime and 200 lb. of quartzite into a blast furnace. The charge thus contained 200 lb. tin, 3050 lb. of zinc and about 4800 lb. carbon.

Of the 200 lb. of tin 13.4 lb. that is 6.7% by weight of the total, was found in the slag. The balance was volatilized.

In this furnace operation, the amount of tin volatilized was about 6.4% of the weight of zinc volatilized and about 3.9% of the weight of carbon burnt.

Preferably the tin and zinc together are condensed in metallic form.

To this end the charge was preheated to 800 C. the air blast to 600 C. or higher. Oxygen-containing gas was introduced into the blast furnace oiftake gases to raise their temperature as described in my United States Letters Patent No. 2,682,462, patented June 29, 1954, and a molten lead splash condenser was used for condensing the gaseous vapours as described in the United States Letters Patent of Robson and Derham Nos. 2,668,047 and 2,671,725, patented February 2 and March 9, 1954, respectively.

The volatilized tin is formed by the reaction between stannous sulphide vapour and either zinc vapour or zinc in solution in the molten lead in the condenser, as represented by the following equation:

All of the volatilized tin is dissolved in the molten lead in the condenser. When the molten lead is cooled outside the condenser, tin-containing zinc separates therefrom, and the residual molten lead is recirculated to and through the condenser.

Continued operation of the process results in the tin content of the lead in the circuit building up, with the tin content of the zinc that is separated increasing proportionately; a final steady state is reached with the circulating lead containing about 4% tin in solution, and the separated zinc containing about 6% tin, once this 2,779,673 Patented Jan. 29, 1

2 state is reached, the zinc-tin alloy separated thereafter contains these two metals in the same ratio as they are being volatilized from the furnace.

The tin is recovered by the distillation of the tin-zinc alloy separated.

Dross production, i. e. the oxidation of the Zinc and lead, was small. Slagging was normal, the slag being fluid, but not melting prematurely. Elimination of zinc from the furnace was also good.

Various modifications may be made within the scope of the invention. Thus it is not essential that lead shall be used for the condensation; zinc, for example, might also be used.

Investigations have now established that under the conditions prevailing in a zinc blast furnace very little tin can be volatilized as stannous oxide, but a considerable amount can be volatilized as stan'uous sulphide. In the customary smelting of tin ores, conditions are not the same as in a zinc blast furnace, but the reducing power of the gases would be of the same general order; accordingly, the fact, that in customary tin smelting of oxidized tin ores, the amount of tin volatilized is small but not quite negligible, is explained by the investigations. Most tin ores are almost free from sulphur; sulphur-containing tin ores are roasted before being smelted. The investigations indicate that small amounts of sulphur in tin ores may, by forming gaseous stannous sulphide, contribute appreciably to the small amount of tin that is volatilized in the customary tin smelting.

The tin, introduced into the zinc smelting blast furnace as stannic oxide, can be regarded as undergoing the following reactions:

SnO2+2CO=Sn(liquid) +2CO2 (2) This absorbs very little heat, and is followed by:

Sn(liquid) +ZnS(solid) =SnS(gas) +Zn(gas) (3) which absorbs 101.9 kcal. per gram-molecule, or 858 C. H. U. per lb. of tin or 1544 B. t. 11. per 1b. of tin.

Experiments have established that the equilibrium constant for Formula 3 is:

K=0.000ll at 1000 C.

0.0001 1/ 0.05 =0.0022 atmosphere The carbon monoxide and carbon dioxide between them occupy about 33% of the total gas volume per atom of carbon burnt in the furnace, therefore atoms of tin could be volatilized as stannous sulphide.

By weight per unit of carbon burnt, there could be volatilized:

X 0g=0.066 unit Since the suggested upper limit of the volume concentration of stannous sulphide is 0.22% compared with 5.0% zinc, the sulphur needed is 0.22/50:0.044 atom per atom of zinc, or 1% of sulphur in the metalliferous charge components. There is also always some sulphur present in the coke. No sulphur was deliberately added for the example referred to, and it is expected that sufficient sulphur would always be present.

if the furnace gas contained 5% zinc and 0.22% stannous sulphide, the tin volatilized would be 8% the weight of the zinc. The carbon dioxide content of the otftake gases might be increased by 0.4% i. e. to a relatively small extent, in the initial reaction between stannic oxide and carbon monoxide (reaction 2 hereinbefore).

Were lead present in the charge, there would be no interaction between lead vapour and stannous-sulphide vapour. Lead-sulphur vapour would tend to react with tin according to the reaction:

PbS(gas) +Sn(liquid) =Pb(gas) +SnS(gas) (4) The invention thus, providesa process for smelting oxidized tin compounds along with oxidized zinc compounds in a Zinc-smelting blast furnace. The smelting charge contains about 1% of sulphur based on the metalliferous content of the charge for forming stannous sulphide vapour. The blast furnace is preferably operated in substantially the same manner as for zinc smelting, as described in my atorementioned Patent No. 2,682,462. Typically, the blast furnace gases withdrawn from the gas offtake of the furnace at a temperature of about 1000 C. contain, by volume, about 5% zinc, about 0.22% stannous sulphide, about 7% carbon dioxide, about 26% carbon monoxide and about 62% nitrogen. The stannous sulphide vapour is reduced to metallic tin (reaction 1. hereinbefore), and the tin is dissolved in the molten lead (or molten zinc) of the splash condenser. Outside the condenser, molten tin-containing zinc separates from molten tin-zinc-containing lead, and the latter is returned to the condenser.

I claim:

In the method of operating a blast furnace in which a preheated charge containing an oXidic zinc material and carbonaceous fuel is introduced into the top of the furnace, preheated air is blown into the bottom of the furnace, molten slag is tapped from the bottom of the furnace, and a gaseous mixture containing zinc vapor, carbon monoxide and carbon dioxide is withdrawn from a higher part of the furnace and passed into a condenser to recover molten zinc, the improvement which comprises incorporating in said charge a substantial amount or oxidic tin material and about one percentof sulphur based on the metalliferous content of the charge whereby under the conditions prevailing in the blast furnace sulphur reacts with oxidic tin material to form stannous sulphide vapor which enters the aforementioned gaseous mixture, reducing the stannous sulphide entering said gaseous mixture by contact with zinc to metallic tin, incorporating said metallic tin in the molten zinc recovered in said condenser, and separating tin from the tin-containing zinc.

References Cited in the file of this patent UNITED STATES PATENTS 2,668,760 Breyer et al. Feb. 9, 1954 FOREIGN PATENTS 367,611 Great Britain Feb. 25, 1932 

